1. Field of the Invention
This invention relates to electronic calculator systems of the type wherein all the main electronic functions are integrated in a single large-scale-integrated (LSI) semiconductor chip or a few such devices, and more particularly, to providing means to protect portable hand-held electronic calculator systems from theft by making such calculators unusable without knowledge of an anti-theft code.
2. Description of the Prior Art:
Perhaps the most popular type of electronic calculator available on the market today is the portable battery-powered type. This type of calculator may usually be held in the hand and fits easily into a shirt pocket. As such, it may easily be stolen and concealed if left lying on a desk or in a laboratory. It may be inconvenient for the owner to lock up the calculator or take it with him whenever he leaves his work area. Some means contained within the calculator itself are needed to protect the device from theft. If calculators were manufactured using discrete components, it would be relatively simple to provide a means for examining the first inputs from the calculator keyboard to determine if they matched an individual anti-theft code stored in the calculator memory. However, most calculators today are manufactured with a single large-scale-integrated semiconductor chip or a small number of such chips.
Electronic calculator systems of the type wherein all of the main electronic functions are integrated in a single large-scale-integrated semiconductor chip, or a small number of chips, are described in the following prior applications which are assigned to the assignee of this invention:
Ser. No. 317,493, filed Dec. 21, 1972 now U.S. Pat. No. 3,819,921 (originally filed Sept. 29, 1967, Ser. No. 671,777 now abandoned) by Jack S. Kilby et al, for "Miniature Electronic Calculator". PA0 Ser. No. 420,999, filed Dec. 3, 1973 and now abandoned, by Gary W. Boone and Michael J. Cochran, for "Variable Function Programmed Calculator", a continuation of Ser. No. 163,565, filed July 19, 1971 now abandoned. PA0 Ser. No. 444,226, filed Feb. 20, 1974 now U.S. Pat. No. 4,200,926, by Michael J. Cochran and Jerry L. Vandierendonck, for "Electronic Calculator Implemented In Semiconductor LSI Chips With Scanned Keyboard And Display", a continuation of Ser. No. 255,856, filed May 22, 1972 now abandoned. PA0 Ser. No. 400,437, filed Sept. 24, 1973 now U.S. Pat. No. 3,987,416, by Jerry L. Vandierendonck, Roger J. Fisher, and Glenn A. Hartsell, for "Electronic Calculator With Display And Keyboard Scanning Signal Generator In Data Memory". PA0 Ser. No. 397,060, filed Sept. 13, 1973, now U.S. Pat. No. 3,900,722 by Michael J. Cochran and Charles P. Grant, Jr., for "Multi-Chip Calculator System".
The concepts of these prior applications have made possible vast reductions in cost of small personal sized calculators.
It would be very costly to design and manufacture a different chip for each individual code since the economy of manufacturing in volume would not then be present. The chip which performs the electronic functions of the calculator must cost only a few dollars in order for the cost to be competitive. This requires high volume production, and it is not practical in such production to make every chip with a different code stored internally. Further, because a given chip design contains only a certain number of words of read-only-memory (ROM) storage for instructions stored internally, it is not practical to store all the possible codes within such ROM storage and select only one for use in each calculator. This limited amount of ROM storage also dictates that the scheme used for the anti-theft feature must not be so complicated that a large number of instruction words of the ROM are used in processing an anti-theft code. Also contributing to the cost of the LSI semiconductor chip which is used in the electronic calculator system is the number of input and output pins, and typically all pins connected to the chip are used to provide the functions of the calculator. Therefore, the scheme used to provide theft protection should not require additional pins, and if additional pins are required, their number must be kept to a minimum.